Find the root attribute

R/D Age Income Student Credit_rating Class
1 <=30 High No Fair No
2 <=30 High No Excellent No
3 31...40 High No Fair Yes
4 >40 Medium No Fair Yes
5 >40 Low Yes Fair Yes
6 >40 Low Yes Excellent No
7 31...40 Low Yes Excellent Yes
8 <=30 Medium No Fair No
9 <=30 Low Yes Fair Yes
10 >40 Medium Yes Fair Yes
11 <=30 Medium Yes Excellent Yes
12 31...40 Medium No Excellent Yes
13 31...40 High Yes Fair Yes
14 >40 Medium No Excellent No

As age is the attribute with highest Gain value, age becomes root of the Decision Tree